The proposed studies are designed to evaluate the effect of fluoride bound to hydroxyapatite on dental caries and to evaluate the efficacy of certain procedures to increase enamel and cementum fluoride acquisition. In the first section 200 pairs of contralateral premolars extracted as part of orthodontic treatment planning were collected from 10- to 13-year-old children resident in areas with optimally and nonfluoridated drinking water. Enamel biopsies were performed on the smooth, exposed facial and lingual surfaces and the shielded approximal surfaces of these teeth. Five successive biopsies, each approximately 5 Mum deep, were performed on the facial and mesial surfaces while five successive biopsies, each approximately 1 Mum deep, were done on the lingual and distal surfaces. In the second section the enamel and cementum fluoride uptake from an adhesive fluoride-containing dental restorative material, the glass ionomer cements, will be evaluated. The already high prevalence of root surface caries observed in some recent epidemiological studies can be expected to increase in the foreseeable future. The treatment of these lesions will present a tremendous challenge to the dental profession because these lesions are difficult to restore and are prone to develop secondary caries. In the third section the effect of contact time of topical fluoride agents on enamel fluoride uptake will be studied. In this in vitro study enamel fluoride acquisition, distribution and retention from Acidulated Phosphate Fluoride, and the fluoride-containing varnishes Fluor Protector and Duraphat will be determined. Fluoride retention after suspension of the treated teeth in synthetic saliva for periods up to six months will be determined and selective KOH extraction will be used to differentiate between the reaction products. In the fourth section the effect of pre-eruptive and posteruptive fluoride administration on enamel fluoride uptake and caries inhibition in the rat will be determined. The experiment has been designed so that the effects of fluoride supplemenatition to lactating dams, pre-eruptive administration via tube-feeding without and with posteruptive fluoride supplementation, and posteruptive supplemention during or after enamel maturation in the oral environment can be studied.